I know it's called "cold tail," but I am not so certain this condition is caused so much by the temperature but by exertion. One of my good friends and training partners had one of his dogs come down with "cold tail" two years ago after the 4th of July weekend. It was plenty warm obviously. The friend has a lake cabin and the dog had spent three days in and out of the lake and had done a lot more swimming than he was accustomed to. I think it took about two weeks for his tail to come back to normal. That experience leads me to believe the condition is more a result of exertion rather than temperature.
I could believe that it got its name as it comes up during the fall months when temps drop and we are out duck hunting. Probably not so far fetched to say that many more retrieves during the duck season happen in the water than on land and dogs are doing a lot more swimming than they do the rest of the year. You may religiously do a water series or two every time you train during the summer, but that is a lot different than being out in the duck blind day after day making 6, 12, or more retrieves one right after another. Their tails are working more than usual and are also meeting a lot more resistance in the water than the tail would see during the summer training season. I suppose the temp could be part of the problem, just like football players may suffer more muscle and ligament injuries during a game played in December or January than in August or September.
So the suggestion above that your dog's tail may not be conditioned is not necessarily a slam on the overall condition of your dog or a shot at you for taking some old, fat, out of shape dog hunting, but purely a legitimate suggestion at the root cause of your dog's condition. And I would also suggest that perhaps no "tail and bum warming device/vest" would aid in preventing the onset of the condition.
Perhaps someone with better medical knowledge or life experience can chime in. I am just speculating here. Please correct me if I am wrong. I have never had a dog with this issue, but I have goldens and their tails are always working in overdrive because their lives are so absolutely positive all the time. (Before you crucify me, that wasn't a slam against all of you non-golden owners that your dogs are mistreated and also don't have terribly happy lives)